05 November 2014

It may well be the longest and most complex project I've ever embarked on but the results are emphatically worth it. Although I am primarily tasked to investigate the geologic history and evolution of icy bodies across the Middle Solar System (considering the distant Kuiper Belt of small icy objects, the giant planets are more correctly in the middle zone), that work has some extra benefits. The mapping of topography and shapes of features requires precise knowledge of their locations on the surfaces of the icy moons. As I like to do things like this to completion (call it ADD or OCD if you like), the result is a complete set of updated camera vectors for all relevant Voyager, Galileo, and Cassini images of icy satellites. More on camera vectors in moment but the benefit of this is that all these images can then be accurately reprojected to any map format and combined to make a true global map of each body. Using color images as well and you can make a color map!

The first bonus of all this work (and I do mean months of hard labor over a keyboard) was the Atlas of the Galilean Satellites (P. Schenk, Cambridge Univ. Press, 2010) showing global and high-resolution maps of each of the 4 large moons of Jupiter known since Galileo first discovered them in 1610. This volume includes the first and only fully registered and accurate positioning of all the Galileo images of these moons, and each mosaic is faithfully reproduced therein. It is recommended to anyone interested in these bodies, in planetary imaging, and in the naked beauty of the Universe.

I also recently released the updated color map of Triton (see my previous 2 posts), and now the same has been done for the 6 largest midsize icy moons of Saturn [Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus] known before the Space Age began in 1957. (I would like to do Hyperion and Titan in this way but am doubtful I will be able to get to it before upcoming events overwhelm.) These are the maps that, after 18 months of work were released yesterday on NASA Photojournal and the LPI Main Website, and described in detail in an article in the latest Planetary Report. These new maps are the most accurate in terms of location, the highest resolution, and the first to show both albedo/brightness variations realistically and the first to be in full color. Not only that, they reveal these worlds to have a beauty all their own (as described in the Planetary Report article).

Cover of the Fall 2014 issue of Planetary Report, showing part of Enceladus. I should open an art gallery . . .

Getting feature locations in planetary images is a complex business. So before getting into the maps I will attempt to explain. The images come down from space with information about the exposure, including the time and position of the camera (i.e., the camera vector), as well as spacecraft location and other things. This information is the instructions given to the camera, but the spacecraft always has a teeny bit of wobble and the information is always slightly inaccurate as a result. Once sufficient number of images have been built up to cover most of the surface, someone (such as myself) can then go in and select a bunch of match-points that identify features in multiple images. Each points should all have the same location in each image but do not due to the wobble. Once cataloged, the pointing vectors are adjusted (a 'bundle-block adjustment') in a least-squares program until the differences in the match-point locations are minimized. Ideally these difference should be zero but seldom are. Anyway, this new information is then passed back to the images and we can then know precisely where features are.

Why is this important? Obviously we want accurate maps of planets so we can send landers to the right place, and make future observations of changes or unusual features, but we also want to make accurate topographic maps from stereo images and such and that requires accuracy. Scientific work on geologic processes also requires accuracy in position or we get the wrong answer and waste time and money. And if we have inaccurate pointing information our maps are misaligned and we can't make the kind of mapping product like those released yesterday.

An example of a misaligned map (left) and an accurately aligned map (right). Courtesy USGS.

So, this brings us to the new maps. As noted above, these are the best maps produced to date of these objects. They will be updated periodically as our understanding of their rotation state improves and as the last sets of images come through in 2015, but positionally they will not change much more if at all. Several close encounters of Tethys, Enceladus, Dione and even a few more shots of Mimas are on tap for next year. The maps are at different resolutions because the bulk of images for each satellite were obtained at different resolutions because of the Cassini tour geometry and speeds and the size of the objects. The goal was to make maps at the highest resolution possible with as uniform a resolution as possible. I (slightly) favored resolution in each case, and the result was 250 meters for Tethys and Dione, 400 meters for Rhea and Iapetus, 200 meters for Mimas, and 100 meters for Enceladus, which has been the focus of numerous Cassini encounters and is the best mapped icy body in the Solar System.

These are the first global maps to realistically show brightness variations across the surface. Hence you can see the really dark trailing hemispheres of Tethys, Dione and Rhea very well. Bright lineations on Dione and Rhea also stand out as do various bright and dark features such as rayed craters. Like all maps, compromises were required to get a uniform map product, as each image was acquired under its own uniquely different lighting and viewing conditions. When a choice was required I usually chose feature definition (from shading) over brightness variation, for example.

Tale of Two Hemispheres. These global projections show how different Saturn's icy moons can look, depending on the view. The top is the leading hemisphere, covered in smooth deposits and sinuous rilles. Note the young bright ray crater Creusa near top. The bottom view is of the darker heavily cratered trailing hemisphere, which is scarred by arcuate young fracture networks.

The other new feature of these maps is that they are the first accurate maps in color (I think somebody may have done preliminary color maps elsewhere but they are not as complete or accurate as these positionally or in color registration). These new maps are in 'Superman' colors, just beyond the range of normal human color vision. The color choice was not made to annoy anybody. Cassini did obtain some images in the R-G-B range of the spectrum close to human vision but these are insufficient images to construct global maps at high resolution. The natural visual colors of these bodies do reveal information but they tend to be rather bland. Cassini did obtain routine higher resolution coverage of these moons in the near-IR and the UV wavelengths and these are used to make the global maps. "Dialing up' the colors to include these spectral ranges also brings out color contrasts between geologic features much better than the olde R-G-B range.

In addition to the global maps, Cassini obtained a number of higher resolution mosaics, many of them in color. Some of these are shown in the Planetary Report article. This will be the topic of a future blog.

It is all well and good to use maps like these for scientific investigations. That is why we go there, to learn about how the Solar System works. But sometimes it is worth stepping back for a few moments and marveling at the amazing Universe we are part of. Each world out there is unique and holds numerous discoveries and surprises (check out the Planetary Report issue for some of those, but if you search earlier blogs here, and also our 2011 Icarus article where I describe them as well.) These worlds are also little jewels in a vast empty Cosmos, fascinating and wonderful to behold. I hope to have more on these maps soon, but for now enough blubbering! The maps are released to the public to enjoy for free. After all, this is YOUR space program!

To view and download the maps, go to the LPI or JPL websites (The JPL releases will have been dated 2014-11-04 they have scrolled of their page). The maps are released in global and hemispheric views, and with and without annotation, suitable for wall poster printing! The global map can be dropped into GoogleEarth or similar global rendering software. We have released on the LPIwebsite moves showing these moons in rotation and flyby. We are working on how to make them downloadable. In the meantime, they are also on the LPI YouTube channel for quick viewing (other related high-res videos can be found on my galsat400 YouTube channel).

P.S.

If you plan to use them in publications, productions, or presentations, the proper credits are:

Global map(s) of Saturnian moon(s) [name of moon(s)] were produced by Dr. Paul Schenk (Lunar and Planetary Institute, Houston TX. Image data are from the Imaging Science Subsystem (ISS) camera on the Cassini orbiter (NASA, JPL).

First, a correction. The surface compositions of Triton and Pluto are indeed similar but not quite identical. Triton has nitrogen, methane, carbon dioxide and carbon monoxide on its surface, and probably some water ice, but of those ices Pluto does not have carbon dioxide or water ice that we can measure from Earth. What those differences may mean for geologic and atmospheric history no one can say as yet with confidence, but all the more reason for going to Pluto and someday back to Triton.

As a matter of personal opinion, I am sometimes asked which planets I’d like to see explored next. Europa is first on the list, but after that we have the ice giant planets Uranus and Neptune and their strange families of icy moons (including Miranda, Ariel, and Triton to name a few). These large bodies are distinct and different from the gas giants Jupiter and Saturn but have been visited only once, by Voyager 2 with instruments designed in the 1970s. What we could learn by going back has been amply demonstrated by the innumerable discoveries of Cassini at Saturn.

Triton, whose surface may be younger than a few million years and may be geologically active today, is one of the most fascinating bodies on the Solar System. Its maximum surface temperature is only 35 degrees above absolute zero, and yet volcanoes and geysers have remade its surface, possibly within the lifetime of the human species. Even the Voyager scientists, who had become accustomed to surprises after the discoveries on Io, Ganymede, Titan, Miranda and the rest, were left almost speechless as Voyager made its final planetary visit. As Larry Soderblom exclaimed at the press briefing when he showed the first Triton images, “What a way to leave the Solar System!”

T-shirt printed up to during the Neptune encounter 1989. The t-shirt and owner are now 25 years older.

Neptune was fabulous too with its strange and dynamic cloud patterns and its odd, incomplete ring system. One of my first efforts in serious image processing was to reconstruct the Neptune ring high-phase-angle observations. These were the best images of the rings we got, but the long exposures saturated Neptune itself and created bright haloes that were difficult to suppress. Normally exposed Neptune crescent images were substituted but the heavy filtering required for the bright haloes also enhanced noise in the images. The end result was a montage showing a crescent Neptune and the entire ring system. This was done back in 1992 or 93, so I’m sure I or someone else could do a better job now. It is a composite of 5 (or 6?) different exposures taken at different times and distances from Neptune, but all the data are real.

Crescent mosaic of Neptune from Voyager 2 on departure, August, 1989.

Triton Map: Enhancement and 'Color'. The enhancement applied to the Triton map in the August 21 post was a modest contrast-stretch only; no differential color enhancement was applied. Surface brightness contrasts on Triton exist but are not as strong as on Pluto. The color does have a greenish cast in equatorial areas. This seems to be real, but there are ‘concerns’ with Triton’s color. First, the color images were sometimes smeared or noisy, due to long exposures under very low solar lighting intensity for which the cameras were not designed. This explains some of the splotchy color mottling that is apparent in a few areas. Secondly, there are some uncertainties in the photometric properties of Triton. Earth-based spectra of Triton obtained in the 1970’s and 80’s differ in the inferred visual color of Triton and it was not possible to get an exact color ‘calibration’ on Triton. We did our best, but the colors may not be only approximate, given the slightly different color sensitivity of the Voyager 2 camera.

The Triton map is suitable to drop into Google Earth or similar programs! You can now zoom and spin on Triton in any way you like.

Neptune in the Movie. Several have asked why Neptune doesn’t appear in our movie. Several reasons, the most important of which is that we ran out of time for the August 25 anniversary. The second is that we compress almost 10 days of the encounter into 1 minute. Neptune would probably appear in 2, maybe 3 of those frames. We are looking into it. We know that Neptune and Triton do appear together in the sky about a day out from Triton, and again 6 days later, but do not appear in proximity to each other on the way in, apparently. We may attempt to add Neptune back in for a final version later this year.

21 August 2014

Triton at +25, Pluto at -1: Twin Planets Separated by Gravity

[An addenda and errata for this post has been uploaded on Aug 23. Click here to go to it and read more.]It has been quite a long time since my most recent post but it doesn't mean I haven't been busy! I have been working long hours preparing a new set of global maps of icy moons, the first of which is being released today. This is the new high-resolution color map of Neptune's large and crazy moon Triton (The next set will be released within a month, and the Galilean Satellite global maps were released in the Atlas of the Galilean Satellites in 2010.)

August 25, 2014 is an interesting date in Solar System exploration. It is foremost the 25th anniversary of the Voyager 2 encounter with Neptune and Triton. This was the grand finale of that landmark mission, which over the span of 10 years completed the first exploration of the giant Outer Planets. It is thus a good day to release the new Triton map.

I was a freshly minted post-doc at JPL in 1989, having arrived the year before. Having been a summer intern for Voyager Science Support under Dr. Ellis Miner 10 years before during the Voyager 2 Jupiter encounter, and then 2 years later for Saturn, I felt a bond with the mission and its support teams. Many of these people I know today! But in 1989 I had no connection with the Project. Fortunately I knew Bill McKinnon, my thesis advisor, and he knew folks on the Imaging Team. In the spirit of celebration surrounding the Neptune encounter, the two of us were snuck into the Inner Sanctum in Building 264, third floor, where I had been an intern 10 years before. (True, 'someone' objected to us being there but we were snuck back in anyway and no one else complained. It was a grand and special event and [almost] everyone was happy to be part of it and share it with a few colleagues.)

One of the few shots of myself (back left) during my Voyager internship during the Jupiter Summer of 1979. Project Scientist Dr. Ed Stone is in front of me at the head of the conference table.

Here are some of the photos (on film!) I took during the Neptune encounter 10 years later . . .

Bill McKinnon, and some of the TV crews assembled to report the Neptune encounter, 1989.

A JPL billboard showing Voyager's progress.

Jay Inge making an airbrush map of Triton, the last body mapped in this way.

Lastly, that's me in front, with Jeff Moore right behind. What a grand time! Thanks to the Voyager Project for letting some of us in to share it.

I along with everyone else at JPL in those days was treated to the Voyager Neptune encounter live every day on our closed-circuit TV monitors (this was just a few years before the WWW, cell phones, etc.). Each day Neptune and Triton got a little bit bigger and more detailed as new images were flashed to the monitors. The fantastic cloud patterns of deep blue Neptune were fun, but for Triton, we didn't even know how large it was, so everything was new. It wasn't until a week before encounter that we learned the radius of Triton for the first time. The surface itself finally became clear only on August 24, the day before closest approach, when Triton's strange features became distinct. The new map being presented today is the product of those images.

The Voyager Project also published a Travel Guide (before pdf's!) describing the Neptune encounter. It contains many gems of wisdom and fun facts. Included are the following quotes from a section in which Voyager 2 leaves a hypothetical diary during its trip out of the Solar System.

Note the little 'flip-movie' of the encounter at bottom left.

"Star Date -1.259 (1990) Today I am filled with an intense sorrow. The encounter with Neptune has ended. It is a but a tiny star in the background. Part of my sorrow is from not being able to visit Pluto before my departure, for it is a very mysterious body. . . . I wish that I, or my sister spacecraft, had been the one to unveil some of the mysteries of Pluto. The Project never seriously considered a visit to Pluto because it would have meant foregoing Voyager 1's encounter with Saturn's moon Titan. . . . However, it will be many decades before man will send another spacecraft out to that part of the solar system." (!)

Like Voyager 'said,' one of the things left undone was the exploration of the trans-Neptunian realm. Voyager 1 was going too far north, and Voyager 2 too far south. Other than Pluto itself we had no knowledge of the "Kuiper-Edgeworth Belt" or just Kuiper Belt, until 1992, when the first non-Pluto object was discovered in that zone. Since then hundreds more icy objects have been found. These objects are smaller than Earth, have icy surface compositions, and extend well beyond Neptune. New Horizons is the first dedicated exploration of that zone. It crosses the orbit of Neptune on August 25, 2014, on the same day as the 25th anniversary of Voyager passing Neptune. It is also now a mere 11 months from Pluto, its main target, which it will reach in mid-July 2015.

In a double sense this is fitting, as Triton is a near twin of Pluto. Triton and Pluto are both slightly smaller than Earth's Moon, have very thin nitrogen atmospheres, frozen ices on the surface (carbon monoxide, carbon dioxide, methane and nitrogen), and similar bulk composition (a mixture of ices, including water ice, and rock. Triton however was captured by Neptune long time ago and has been wracked by intense heating ever since. This has remade its surface into a tortured landscape of overturned layers, volcanism, and erupting geysers.

What will we see at Pluto? Guesses have ranged from active geology to cold and cratered, so we are in for a suspenseful Summer next year! Triton is of importance as it offers clues to what geologic features might look like on Pluto, given that the icy crusts of both bodies are probably rather similar and would presumably react in similar ways under internal stress and heat. So if there were or are volcanoes on Pluto they could look similar to those we see on Triton.

So what does the Triton map show and how was it constructed? The Triton map was constructed from images acquired in the clear, orange, green and blue filters (ultraviolet filter images were also used and this map will be posted at a later date). The images were co-registered in an updated control network, which determines where the camera was pointing in each frame with precision so that feature geography is well known. Photometric adjustment of each image was also done so that shading due to the curvature of the surface can be corrected. The map also includes the departing crescent imagery, which is apparent in the left hand part of the map. I hope to post the actual mosaic components in a future post.Go Here to Find Full Resolution Triton Map and a Movie Recreating 1989 Voyager Flyby (the video is large and may take a few minutes to download):

Triton global map (scaled-down by a factor of 16!). Dark areas were in shadow or not observed.

[A Note on the Use of the Triton Map and Video:
All the Triton maps and videos are in the public domain and may be used
freely. Credit should be notes as: Triton map produced by Dr. P Schenk, Lunar
and Planetary Institute, Houston. Triton video produced by Dr. P. Schenk and J.
Blackwell, Lunar and Planetary Institute, Houston (or LPI may be used if short form
desired). Use of map and video does not constitute an endorsement of any other product.]

The result is a smooth map showing uniform shading and reasonably accurate brightness and color representation. Note that the orange filter used by Voyager is not quite the same as our human optical red sensors but is close enough. Although the map approximates natural colors, there are some uncertainties in the color calibration and the "true colors" of Triton in 1989 may be slightly different.

In the intervening quarter century and its many discoveries, I think we have tended to forget how strange and exotic Triton really is! Its effective surface age may be a little as 10 million years, clearly implying that active geology is going on today. The cantaloupe terrain, which I interpreted back in 1993 as due to crustal overturn (diapirism), hasn't been seen anywhere else. The volcanic region with its smooth plains and volcanic pits large and small, is the size of Texas. And the southern terrains still defy interpretation.

The Triton map also gives a sense of the quality of the Pluto map we hope to get. The Triton encounter was rather similar to our upcoming Pluto encounter in that New Hoizons will zip through the Pluto system at a high speed, a leisurely 11 kilometers per second compared to 25 km/s for Voyager at Triton, and both bodies rotate rather slowly, ~ 6 days. This means that for both bodies we will have seen one side well at high resolution and the other side at much lower resolution, roughly 25-to-40 kilometers. The northern polar regions of Pluto and Charon will also be in darkness as seen by New Horizons, as they were for Triton.

One important difference is that New Horizons carries more powerful remote sensing instruments and will obtain infrared spectroscopy that Voyager, built in the early 1970's, could not. We should be able to map the distribution of ices across the surface. We will also resolve features several hundred meters across large areas and as small as ~90 meters in smaller regions of Pluto. Voyager's best resolution on Triton was ~300 meters in a limited area. This should be more than enough to map crater distributions, volcanoes, faults, and erosional processes. It will also be sharp enough to see if Pluto has any atmospheric plumes (or geysers) like Voyager saw at Triton. These can be seen as thin dark and bright streaks on the Triton map.

Producing global maps of Pluto and Charon will take some time as the playback of data from that great distance will be rather slow. It will take most of next autumn to return all the images and data from Pluto so we will have to be patient. The end result should be maps of the surfaces of Pluto and Charon even better than we have for Triton. It will be most interesting to compare Pluto with what we saw at Triton to see if there are any similar features, and to see whether or not Pluto has ever been geologically active.

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NOTES ABOUT THE DATA

These brief notes are posted to help viewers understand the nature of the topographic data. Voyager, Galileo and Cassini carried imaging cameras but not altimeters. Topography is instead generated using stereo images or low-Sun images (which can be used to calculate slopes and heights). Neither method is perfect and often, as is the case for Miranda, individual stereo pairs must be constructed and then stitched back together to form a global or partial topographic map. This means that the elevations shown are not precise with respect to the center of the body. Height values derived are accurate however with respect to local features. For example, we know the steep cliff on Miranda is about 10 km high top to base but we do not know how high it is with respect to the mean "sea level" on Miranda. This cannot be determined until we return to these places.I will post additional information on the data over the next few days.

Inquiries for the scientific use of the original digital elevation data should be forwarded to schenk@lpi.usra.edu.